Download Rembrandt: The Late Works - Science Teachers Word Bank

Rembrandt: The Late Works
Science Teachers Word Bank
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Atomic number (Z)
The atomic number of a chemical element is the number of protons found in the nucleus of
the atom. Lead, with an atomic number of 82, is regarded as a heavy metal and is present in
pigments such as lead white.
Catalyst
A catalyst increases the rate of a chemical reaction, without itself being consumed in the
reaction. Certain metal compounds can act as a catalyst in the ‘drying’ (polymerisation) of oils
and may have been added to the paint deliberately for this purpose. Some pigments can also
act as catalysts in the polymerisation of oil and are therefore known as ‘good driers’.
Conservation
The main activities of conservators are cleaning, restoration, research, contributing to
scholarly and exhibition catalogues, and ensuring that pictures lent and borrowed by the
Gallery are in a sound state.
Cross-Section
A cross-section can be made from a tiny paint fragment or sample, which has been removed
from a painting. The fragment will usually contain several paint layers and will be embedded
in a block of polyester resin, then ground and polished to reveal its edge. This cross-section
can then be examined under an optical microscope, which generally has a magnification
range of 60 to 1000 times. By examining the paint fragment in this way, the layer structure of
the sample can be observed. This information can be used to better understand how the
painting was constructed. Many of the pigments seen in a cross-section can be identified
using an optical microscope. More detailed information can be obtained by analysing it using
a scanning electron microscope (SEM) with an energy-dispersive X-ray (EDX) detector.
Gas Chromatography – Mass Spectrometry (GC-MS)
Gas chromatography – mass spectrometry is a technique for the separation and detection of
chemical components from complex mixtures. It is suited to the analysis of small quantities of
material and is therefore ideal for providing detailed chemical information on the organic
materials found in paint samples. It is particularly valuable for the identification of natural
products such as oils and resins, where specific molecular indicators can distinguish between
different species and can also be used to distinguish the different drying oils in paint samples,
such as linseed and walnut oil.
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Energy Dispersive X-Ray (EDX) analysis
This technique is a method of elemental analysis and is often carried out in the scanning
electron microscope (SEM). A single paint layer in a cross-section, or an individual pigment
particle of very small size, can be analysed for its constituent elements.
Fourier transform infrared (FTIR) Spectroscopy
Infrared spectroscopy is usually employed to identify the organic materials present in paint
samples, such as the paint binder or varnish, although some pigments can also be identified
using this technique. The molecules present in a sample absorb infrared radiation at specific
wavelengths, producing a characteristic ‘fingerprint’ for each class of material. This technique
uses a beam of infrared radiation containing wavelengths in the mid-infrared region (2,500
nanometres to15,400 nanometres) and is different from infrared reflectography.
Impasto
Impasto is a style of painting where the paint is put onto a surface thickly and with visible
brushstrokes and texture.
Infrared image (Infrared reflectography)
Near-infrared radiation (with a wavelength of between approximately 750 nanometres – 2500
nanometres) is often able to penetrate paint layers which are opaque to the human eye. The
radiation passes through the paint until it reaches something that absorbs it, such as carbon
black, or until it is reflected back to the camera. An image is formed by the different
absorption and scattering properties of the paint at a given area of the painting. Infrared
reflectography is most commonly used to see underdrawing – the design that the artist
draws, usually in black, before beginning to paint.
Lead white (basic lead carbonate)
Lead white, or basic lead carbonate, is an opaque white pigment. It was the main white
pigment used by artists until the 19th century and was produced artificially by exposing
metallic lead to acetic acid in a carbon dioxide-rich environment. The pigment also acts as a
catalyst in the ‘drying’ of oil paint.
Octahedral geometry
Transition metal complexes with an octahedral geometry have six ligands symmetrically
arranged around the central metal ion. The colour of many transition metal complexes is
related to the size of the band gap in the d-orbitals when they are split into two sets. The
molecular geometry will alter the size of this band gap and will affect the colour of the
complex.
Paint Binder (or Paint Medium)
In paintings, a binder (or medium) is the material that holds the coloured pigments together
to form paint. In paintings at the National Gallery this is often a drying oil, but may also be
egg yolk or animal glue.
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Photo-oxidation
Photo-oxidation reactions require oxygen (or ozone) and light and therefore often take place
close to the surface of a painting, such as in a varnish layer. The generation of oxygenated
species can often lead to reactions which result in this layer becoming discoloured and brittle.
The incorporation of oxygen into the resin molecules of a surface varnish will increase their
polarity and will alter the solubility of the layer.
Pigment
Pigments are the colouring component of paint and can be made from a wide range of
materials. These include minerals, natural and synthetic dyestuffs, and other manmade
compounds. A pigment is a powder and is not soluble in the paint binder.
Polarity
Polarity refers to a separation of electric charge leading to a molecule or its chemical groups
having an electric dipole. Molecular polarity is dependent on the difference in
electronegativity between atoms in a compound and the asymmetry of the compound's
structure. Polarity underlies a number of physical properties including surface tension,
solubility, and melting- and boiling-points.
Polymerisation
Polymerisation is a chemical reaction where monomer molecules join together to form
polymer chains or three-dimensional networks. The ‘drying’ of oil is a complex process, but
involves the double bonds of the unsaturated fatty acids in the triglyceride molecules (the
monomer units) reacting with oxygen in the air. These triglyceride molecules can then link
together, forming a three-dimension network. In paint, where an oil binder has been mixed
with pigments, these cross-linking reactions can sometimes be catalysed by certain pigments.
This might affect the speed and type of polymerisation reactions which take place and
therefore the physical and chemical characteristics of the paint film that is produced.
Sample
A tiny amount of paint is sometimes taken from the painting using a scalpel. The paint
scraping or fragment is usually removed from the very edge of the painting or from next to an
existing area of damage. It is so small that it can be difficult to see with the naked eye and
must be examined using a microscope. If the sample is to be used to study the layer structure
of the painting, it may be made into a cross-section. To identify the paint binder, the sample
is prepared in a different way and analysed using Gas Chromatography – Mass Spectrometry
(GC-MS) or Fourier transform infrared (FTIR) Spectroscopy.
Saturated and unsaturated fatty acids
A fatty acid is a carboxylic acid with a long, aliphatic chain. This chain is either saturated (it
has no double bonds) or unsaturated (it contains double bonds). In oil paint, fatty acids are
obtained from triglyceride molecules. The most common fatty acid found in linseed oil is
linolenic acid, which has 18 carbon atoms and three double bonds.
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Scanning Electron Microscopy (SEM)
The scanning electron microscope (SEM) is an instrument capable of magnifying objects up to
100,000 times – far greater than is possible with an optical microscope. There is no colour in
SEM images because they are not produced using visible light. The SEM uses a focused beam
of electrons to scan the surface of a sample under examination, generating several types of
signal that can each be collected by a different type of detector. The SEM is often used
together with Energy Dispersive X-Ray analysis.
Smalt
Smalt is a blue glass, ground into a powder. It was used as a pigment from the 16th to the
18th centuries. The blue colour comes from cobalt ions in the glass linked to oxygen ions in a
tetrahedral geometry.
Soap
A soap is a metal salt of a fatty acid. In paint, the fatty acids derive from the triglyceride
molecules of the oil and can react with the potassium ions leached from smalt to form soaps.
Solvent
A solvent is a substance that dissolves a soluble material (or solute). For example, varnish is
insoluble in water but might dissolve in propanone or acetone. The solubility of a substance
depends on many factors, but a solute will dissolve in a solvent that has a similar polarity
(‘like-dissolves-like’).
Support
The support is the term used to describe the material on to which the artist has painted. It is
usually a canvas or wooden panel, but other supports include copper or stone.
Tetrahedral geometry
Transition metal complexes with a tetrahedral geometry have the metal ion located at the
centre and four ligands located at the corners of a tetrahedron. The colour of many transition
metal complexes is related to the size of the band gap in the d-orbitals when they are split
into two sets. The molecular geometry will alter the size of this band gap and will affect the
colour of the complex.
Triglyceride molecules
Triglyceride molecules are esters of glycerol (a trihydric alcohol) with a range of different long
chain fatty acids. Fats and oils are defined as mixtures of ‘mixed triglycerides’ that is each
triglyceride molecule can contain different long chain fatty acids. A ‘drying’ oil has a high
proportion of triglyceride molecules containing unsaturated fatty acids.
Varnish
A varnish is a clear, usually colourless coating, applied to bring out the colours of paintings
more intensely (improve the saturation) and to protect the surface. Many different types of
varnish exist, but traditional varnishes used by artists and conservators are made from natural
resins, obtained from certain types of trees. These resins are dissolved in a solvent, which then
evaporates leaving a clear film on the surface of the painting.
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Viscosity
Viscosity is a measure of a fluid’s resistance to deformation by stress and corresponds to a
liquid’s thickness. In general, the more pigment there is relative to binder, the thicker the
paint and the more viscous it will be. Sometimes the drying oils themselves were thickened. A
viscous paint with specific characteristics could then be produced using these thickened oils.
X-ray image (X-radiography)
X-rays can pass through most solid objects, but the greater the density of a material, the more
it is able to impede X-rays. Some pigments contain metal ions that have a high atomic
number (Z) and can be considered to have a high density. These pigments impede X-rays
better than pigments containing chemical elements with low atomic numbers, so any areas of
paint containing a large amount of such pigments will appear white in an X-ray image.
X-radiography is useful for revealing the structure of a painting and any changes that may
have occurred at different stages in its development.
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